1. Field of the Invention
The present invention relates to a photosensitive image-forming material and a photosensitive composition, which can be used as an offset printing master. More particularly, the present invention relates to a positive type photosensitive image-forming material for use with an infrared laser and a positive type photosensitive composition for use with an infrared laser, which are suitable for the so-called direct plate making and which are capable of directly making a printing plate using an infrared laser based on digital signals outputted from a computer or the like.
2. Description of the Related Art
Recently, the systems of direct plate making from digital data of computers have been attracted public attention, and the various techniques have been developed. As the systems of direct plate making from digital data of computers, (1) a system using an electrophotography, (2) a system using photopolymerization wherein an Ar laser exposure and an after-heating is carried, (3) a system formed by laminating a silver salt layer on a photosensitive resin, (4) a system of a silver master type, (5) a system formed by breaking a silicone rubber layer by the discharge breakage or laser beam, and the like, have been known.
However, in a (1) system using an electrophotographic method, image forming processes such as electrification, exposure, development and the like are complicated and, therefore, apparatus becomes complicated and large. In the method (2), since a post-heating step is required and the highly sensitive printing plate is utilized, it becomes difficult to handle the plate in an illuminated room. In the systems (3) and (4), those methods have a disadvantage such as the complicated processes and a high-cost since a silver salt is used therein.
The method utilizing a system (5) is a method which have a high level of performance comparatively, but have a problem of the removal of a remaining silicone residue on the print plate.
On the other hand, a laser technology has advanced remarkably in recent years and, particularly, high output and small-sized solid-state lasers and semiconductor lasers, which have a light emission range from the near infra-red region to the infrared region, are easily available, and they are very useful as light sources for an exposure when direct plate making is performed through use of digital data.
As a conventionally known positive planographic printing plate material for an infrared laser which is used for direct plate making conventionally known, an aqueous alkali solution-soluble resin having a phenolichydroxyl group, such as anovolak resin, etc. in used. For example, Japanese Patent Application Laid-Open (JP-A) No. 7-285275 suggests image-forming materials in which a compound, which generates heat upon absorbing light to generate heat, and various onium salts and quinonediazide compounds were adding to the aqueous alkali solution-soluble resin having a phenolic hydroxyl group, such as a novolak resin, etc. These image-forming materials form an image like that the portion of a image area can't remove since the onium salt and quinonediazide compound acts as a dissolution inhibitor of the aqueous alkali solution-soluble resin, and the portion of a non-imaged area can remove since the dissolution inhibitoris decomposed by heat and it don't exert a dissolution inhibition capability.
However, there was a disadvantage such that the place for treatment is limited under a yellow lamp in such an image-forming material, because the onium salts and quinonediazide compounds have a light absorption range in the visible radiation range (350 to 500 nm). Further, the developing property in case of using a solution having low developing capability, that is, the developing property in the under conditions is insufficient, for example, a decomposition rate of the heat decomposable material become lower when exposure is performed by using low-power laser.
Furthermore, there was a problem that the onium salts and quinonediazide compounds don't have always good compatibility with the aqueous alkali solution-soluble resin and compound which generate heat upon absorbing light, therefore, it is difficult to prepare a homogeneous coating solution and to obtain a homogeneous and stable image-forming material.
Japanese Patent Application Publication (JP-B) No. 46-27919 discloses an embodiment of forming an image by using a novolak resin without using a photosensitive compound, but there were problems that the novolak resin itself has poor solvent resistance and, therefore, the plate wear resistance is lowered by use of a cleaner and printing can not be performed by using UV ink.